Optical couplers, optical switches, such as 1×2 optical switches, and optical power splitters are needed in many optical applications. For instance, one can combine arrays of 1×2 optical switches to make optical cross-connect switch fabrics. It is especially advantageous if these optical switches are integrated onto the same substrate, saving cost, size, and loss and as such may be embedded in integrated networks of optical interconnections. These optical switches should have low loss and maintain good characteristics in the presence of wavelength, polarization, and fabrication (WPF) changes. Similarly, optical power splitters should also have low loss and maintain good characteristics in the presence of wavelength, polarization, and fabrication (WPF) changes.
There are two main characteristics of 1×2 optical switches that are sensitive to WPF changes. The first characteristic is the switching extinction ratio for both outputs, i.e., when the light is switched to output port 1, how much light leaks into output port 2 and vice versa. The second characteristic is the bias point of the switch, i.e., when no electrical power is applied, how accurately is the switch in one of the switch states.
Typically, in electro-optic and polymer materials, the preferred 1×2 optical switch configuration is usually the “Y” switch configuration. This has an accurate power-off state, but often a poor extinction ratio. “Y” switches consume high electrical power when operated thermo-optically in silica waveguides, and so are limited to only materials like LiNbO3, InP, and polymers.
The other main configuration choice for a 1×2 optical switch is the Mach-Zehnder interferometer (MZI) switch configuration. This configuration consists of two couplers connected by two waveguides, one or both waveguides containing phase shifters. Changing the phase difference between the two waveguide arms by 180° causes the optical switch to alternate from one state to the other. The MZI switch typically has a significantly lower thermo-optic power consumption than the “Y” switch. Conventionally, MZI switches comprise either two multi-mode interference (MMI) couplers or two evanescent couplers. However, MMI couplers have significant loss, resulting in a 1×2 optical switch with typically 1.2 dB loss in silica waveguides. In addition, the power splitting ratio of the evanescent couplers is highly sensitive to WPF changes, and as such the switch extinction ratio for at least one of the ports is highly WPF sensitive.
There are also two main characteristics of optical splitters sensitive to WPF changes. The first is the splitting ratio of the optical splitter. The second is the loss of the optical splitter. Conventional optical splitters may implement evanescent couplers because of the low loss associated with these types of couplers. However the splitting ratio of conventional evanescent couplers is highly sensitive to WPF changes and the ratio cannot be readily electrically adjusted in non-electro-optic materials, such as silica waveguides.